A. Field of the Invention
The invention relates to a precision clamp-on transit-time mass flowmeter. More particularly, it relates to a multi-pulse flowmeter which employs a train of ultrasonic pulses for determining a flow rate. A phase coherency detector is employed for determining the phase and timing of the pulse train received by the receiving transducer An initial setup routine tests a range of ultrasonic frequencies and determines an optimum frequency before flow rate measurements are made.
B. Background Art
Transit-time flowmeters are known, and are the flowmeter of choice for most fluids. See for example, U.S. Pat. Nos. 4,232,548 and 3,869,915. The disclosures of these and all other patents and prior art mentioned herein are expressly incorporated by reference. However, up to the present, transit-time flowmeters have had disadvantages due to their sensitivity to operating conditions as well as the need for close acoustic matching between their transducers and the pipe in which flow is measured
These flowmeters employ transducers for passing ultrasonic pulses through the wall of a conduit and then receiving the transmitted pulses at a point along the conduit spaced from the transmitting transducer, either on the opposite side of the conduit, or on the same side of the conduit. In the former case, the acoustic energy passes directly through the conduit walls and the flowing liquid, whereas in the latter case, the acoustic energy is reflected by the opposite internal wall of the conduit. Circuitry processes the energy received at the receiving transducer and produces a display of flow rate.
The transducers are clamped to the exterior surface of a pipe and spaced along the pipe axis. They transmit and receive ultrasonic energy through the liquid in either the upstream or the downstream direction. Two measurements are made, namely the difference between the upstream and downstream travel time (.DELTA.t) and the travel time in the liquid (t.sub.L). These parameters are measured by circuitry and used to compute the liquid flow rate.
In the system of the '915 patent mentioned above, a first transducer applies a pulse of ultrasonic energy, say in the upstream direction of fluid flow, and the time taken for a given portion of the signal to reach the second transducer is counted by a counter. This counter counts the number of pulses produced by a high frequency clock generator, which runs during this transmission time. The second transducer then sends a pulse of ultrasonic energy in the downstream direction, and the same counter, connected to the same clock generator, counts down until the given point in the return signal is received by the first transducer. The net count remaining in the counter, then, is a function of the difference in time required for the sound energy to go upstream and downstream, which in turn depends on the fluid flow velocity in the pipe.
The '915 patent recognized the problem of ensuring that the time measurement for all up and down cycles is made at the same point in the received signal. It was recognized that, using reasonable clock frequencies, the count difference in a single up-down sequence will be small, and it would be difficult to accurately measure small changes in flow velocity, since a small change in flow velocity might result in little or no change in the number of pulses of the counting clock signalling the processing circuitry.
Hereinafter the term "up cycle" may be used at times to represent the transmission and reception of an ultrasonic pulse in the upstream direction. Correspondingly, a "down cycle" may be employed to denote the generation and reception of a pulse in the downstream direction.
Also, the downstream transducer will be referred to as the "first" transducer and the upstream transducer will be referred to as the "second" transducer. Thus, an up cycle will involve transmission of acoustic energy from the first transducer to the second transducer, whereas a down cycle will involve transmission of acoustic energy from the second transducer to the first transducer.
Accordingly, the '915 patent system employed a relatively large number of upstream measurements, referred to as "up cycles", followed by a correspondingly relatively large number of "down cycles," to form a single count cycle. In this way, it was possible to increase the count difference between the up and down directions in a given count cycle to increase the resolution of measurement. This was useful, but the problem remained of ensuring that the time measurement for all the up and down cycles was made at the same point in the received signal. This problem is complicated, in reality, by the fact that the received signal is complex, containing echoes and other transient effects and varies from instant to instant. However, it was recognized that while the received signal may be complex, the zero-crossover points in the signal remain fairly stable. Accordingly, circuitry was provided to normalize the received signal level, and gate the receiver circuits open when a given instantaneous signal level was reached. Then, the next voltage zero in the return signal was marked as the point at which the time measurement was made.
In the '915 patent, a plurality of up cycles were carried out, followed by a plurality of down cycles, together forming an entire count cycle. Of course, an up cycle, or a plurality of up cycles, ordinarily has a greater length than a down cycle or a plurality of down cycles. A plurality of between 2 and 512 count cycles were carried out in a read cycle (or, preferably, between 128 and 512 count cycles for improved reading accuracy). However, despite the advantages from aggregating many cycles in this way, it is to be noted that the acoustic pulses were each transmitted, detected and measured individually in the '915 patent system.
While the flowmeter of the '915 patent overcame the drawbacks of standard mechanical flowmeters, its accuracy depended on its ability to precisely measure the time interval from the instant an individual ultrasonic pulse is transmitted by the first transducer until the instant it is received by the second transducer. To obtain the desired accuracy, it is important for the received signal, whose wave form is generally a sinusoidal pulse having an exponential envelope, to be detected with reference to the same point within the wave shape, during each upstream-downstream pair of transmissions, or groups of transmissions. In the '915 patent, this was obtained, as discussed above, by detecting a specific zero crossing point within each one of the wave shapes received.
The '548 patent made a further improvement in measurement accuracy. It was recognized that flow readings were affected by systemic noise in the fluid conduit, referred to as "pipe noise". This pipe noise can arise from miscellaneous vibrations from other portions of the equipment, as well as pipe-specific factors such as the ringing of the transmitting transducer, echoes in the pipe, reflections from pipe joints, and so forth. Noise transmitted through the pipe wall from the transmitting transducer to the receiving transducer may not be phase-coherent with the sonic beam transmitted through the liquid, and can combine with the received signal so as to cause a random phase shift of the zero-crossover points of the sonic beam, if the phase of the pipe noise is not the same as that of the received signal. See generally FIGS. 1A-1D and accompanying text in the '548 patent. Other noise may or may not be phase-coherent with the received signal, and may similarly cause phase shifts of the received signal which are more random in nature. A particular problem is that the frequency of the pipe noise transmitted through the pipe wall from the transmitting transducer may be substantially similar to that of the liquid-transmitted ultrasonic pulse, making it nearly impossible to filter such pipe noise out of the received signal. As a result, the zero-crossover points of the received signal are subject to a systemic phase shift which can consistently distort the measurement of both the upstream and downstream transmission times.
In order to overcome the foregoing problem, the '548 patent system measures the upstream and downstream transmission times at each of a plurality of zero-crossover points of the received pulse. Since the probability is low that the phase relationship between the pipe noise and the received signal will be identical at all of the several zero-crossover points, the average effect of the noise for a large number of upstream and downstream transmissions will most likely be reduced.
Note that here again, although the '548 patent may average or combine the measurement results of a number of individual pulse measurements, it still transmits, receives and processes each of the ultrasound pulses on an individual basis.
The System 480 and System 960 transit-time flowmeters manufactured by Controlotron Corp., 155 Plant Avenue, Hauppauge, N.Y. 11788, correspond generally to the '915 and '548 patents, respectively. As just discussed, each of these transmits, receives and processes single or relatively few short pulses from the upstream to the downstream transducer and vice versa and then combines the results of a number of such measurements. Single pulses can be distorted by pipe anomalies and other structural details of the pipe, or by multipath reflections. To avoid this, these commercial systems designate one or more particular points on the received signal curve to determine the upstream-downstream time difference. The measurement is carried out many times and then analyzed statistically, and great accuracy is attained.
Another model, the System 240, optionally transmitted a short train of pulses, but, again, processed them on an individual basis, and was subject to the above mentioned disadvantages. Also known are systems manufactured by Badger Meter, Inc. See U.S. Pat. Nos. 3,935,735 and 4,052,896. These carry out an analog demodulation, and then filter the demodulated product. Such systems do not give excellent performance in the presence of noise.
These systems have employed a variety of transducers. Particularly advantageous is the wide-beam transducer, disclosed and claimed in U.S Pat. No. 3,987,674, which is capable of improving the strength of the received signal and also minimizing signal dispersion. Other transducer structures which may be employed are disclosed in U.S. Pat. Nos. 4,475,054, 4,467,659, 4,425,803 and 4,373,401. The '659 patent system is of particular interest, since it relates to an advantageous metallic transducer housing having a shape which is capable of converting an injected longitudinal sonic energy beam from a transducer crystal into a shear mode beam by internal reflection from a surface of the housing.
U.S. Pat. No. 4,333,353 is also of interest and is incorporated by reference herein, although it is a Doppler flowmeter rather than a transit-time flowmeter, for its recognition of the problems of transmitting energy from the exterior to the interior of a conduit, due to phase cancellation of the transmit signal due to internal pipe wall reflections. Also of interest are the means proposed in the '353 patent for solving these problems.